A rollover judging device judges a possible rollover of a vehicle on the basis of a roll rate (RR, an angular velocity) when a vehicle is rolled and a roll angle (RA, a roll angle) achieved by integrating the roll rate (for example, see JP Patent Publication No. H9-240399). FIG. 5 shows a map for the judgment of rollover. In FIG. 5, a point A represents a stationary rollover limit angle. The stationary rollover limit angle means an angle, at which a vehicle would surely rollover under a stationary state if the vehicle is further inclined.
In FIG. 5, a line Ath represents a dynamic rollover limit threshold value. The line Ath is calculated from the balance between the rotational kinetic energy and the positional energy at the rolling time. FIG. 6 is a schematic diagram showing a state where the vehicle rolls around a starting point O. Here, the inertial moment is represented by I, the angular velocity at the rolling time is represented by ω, the mass of the vehicle is represented by m, the acceleration of gravity is represented by g, the difference in height between the position of the center of gravity G2 after the rolling of the vehicle and the position of the center of gravity G1 before the rolling of the vehicle is represented by Δh, the distance from the starting point O to the center-of-gravity position G1 is represented by r, the shift angle of the center of gravity at the rolling time is represented by θ, and the angle defined by the road surface H, the starting point O and the center-of-gravity position G1 (i.e., the intersecting angle between the road surface H and a line passing through the starting point O and the center-of-gravity position G1) is represented by φ.
When the roll of the vehicle reaches the stationary limit angle and when the kinetic energy E1 (=mgΔh=mgr(1−sin(θ+φ)) exceeds the positional energy E2 (=Iω2/2), it is estimated that the vehicle would rollover insofar as no external force other than g is applied to the vehicle.
The dynamic rollover limit threshold value Ath is set on the basis of the energy balance described above. In FIG. 5, in the case of a lower left area from the dynamic rollover limit threshold value Ath, the vehicle does not rollover even when it rolls. However, in the case of an upper right area from the dynamic rollover limit threshold value Ath, the vehicle rolls over. The rollover judgment of vehicles has been hitherto carried out on the basis of the RR-RA map as described above. The curtain air bag device, the seat belt pretension device, etc. are driven on the basis of the above judgment.
The vehicle rollover type is classified into flip-over, fall-over, trip-over, etc. The flip-over type is such a rollover type that one wheel of a vehicle goes upon a convex-shaped obstacle such as a curb store or the like when the vehicle travels, so that the vehicle rolls over. Contrarily, the fall-over type is such a rollover type that one wheel of a vehicle falls into a recess obstacle such as a groove or the like when the vehicle travels, so that the vehicle rolls over. In the case of these rollover types (hereinafter referred to as “normal type”), the judgment of the rollover can be performed by the conventional rollover judgment as described above.
The trip-over type is such a rollover type that a vehicle rolls over after the vehicle sideslips. In the case of this rollover type (hereinafter referred to as “trip type”), passengers in a vehicle are moved by acceleration in the lateral direction of the vehicle, and thus the passengers are proximate to the interior members of the vehicle (for example, pillars, window glass, etc.). Therefore, it is necessary to make a driving instruction to a passenger protection device at an earlier stage than the normal type rollover. In other words, in the case of the trip type rollover, a response required time (even when the passenger protection device is driven after the response required time has elapsed, it is difficult to protect the passengers) is shorter than in the case of the normal type rollover.
According to the conventional rollover judgment (the above-described map of FIG. 5), when the rollover type of a vehicle is the trip type, it has been hitherto difficult to complete the rollover judgment in time for the response required time. That is, the response required time is located in the lower left area from the dynamic rollover limit threshold value Ath. That is, at the roll time of the vehicle, the vehicle condition exceeds the dynamic rollover limit threshold value Ath after the response required time has elapsed.